A recent study and analysis of various CPUs have uncovered a significant side channel vulnerability in specific AMD processors, now registered under CVE-2023-20569. This vulnerability is particularly unsettling because it enables potential attackers to manipulate the return address prediction of a CPU, forcing it into speculative execution at an address under their control. Consequently, this could lead to severe information disclosure and security breaches. This blog post investigates CVE-2023-20569, provides code snippets, links to original references, and discusses various exploit details.

CVE-2023-20569: The Side Channel Vulnerability in AMD CPUs

The vulnerability present in some AMD CPUs allows potential attackers to exploit the processor's branch prediction mechanism. The impact of this vulnerability is that it grants the attacker control over speculative execution, essentially providing them with the "keys to the kingdom." To understand this attack vector better, let's first take a look at the branch prediction mechanism and how it works.

Branch Prediction Mechanism

In modern CPUs, the branch prediction mechanism serves as a performance optimization technique. Branches refer to a program's decisions (i.e., if or loop statements). By predicting which branch will be taken and executing instructions ahead of time, the CPU anticipates its workload and optimizes performance. However, this same mechanism leaves processors vulnerable to potential side channel attacks, such as those CVE-2023-20569 aims to exploit.

Code Snippet and Original References

To demonstrate the side channel vulnerability, we will employ a code snippet from the original research paper detailing CVE-2023-20569. The source code demonstrates how the attack can be performed, providing valuable insight into the vulnerability.

You can access the original research paper here: Original Research Paper

#include <stdio.h>
#include <stdint.h>
#include <time.h>

#define TARGET_LATENCY 100

uint8_t array[256 * 4096];
uint8_t target_array[256 * 4096];
/* Function that mimics an attacker-controlled address */
void attacker_function() {
    array[5 * 4096] += 1;
}
int main() {
    int i, offset;

    // Initialize both arrays with arbitrary values
    for (i = ; i < 256; i++) {
        array[i * 4096] = 1;
        target_array[i * 4096] = 1;
    }

    // Record the current time
    uint64_t time1 = __rdtscp(&offset);

    // Call the attacker-controlled function
    attacker_function();

    // Record the time again after the function call
    uint64_t time2 = __rdtscp(&offset);

    uint64_t exec_time = time2 - time1;

    printf("Execution took %lu clock cycles\n", exec_time);
    if(exec_time < TARGET_LATENCY) {
        printf("Exploit detected!\n");
    } else {
        printf("Exploit not detected!\n");
	}
    return ;
}

Exploit Details

The exploit essentially triggers a vulnerability by misleading the branch predictor into training it with the attacker's desired return address. Once accomplished, the attacher can exfiltrate valuable information by monitoring cache side effects.

As detailed in the original research paper, the following are ways to exploit this vulnerability

1. Thwart the CPU's constant time-array access, potentially allowing attackers to gather data from the target's cache line.

Bypass CPU isolations and access privileged functions like kernel memory.

It is worth noting that any potential attacker needs to be in a position where running arbitrary code is feasible. This might include environments like shared hosting, VM or browser-based applications.

Employing hardware mitigations for future processor designs.

3. Discouraging deployment of vulnerable processors in shared environments, including server farms and VM pools.

Conclusion

CVE-2023-20569 represents a severe side channel vulnerability affecting some AMD CPUs. This vulnerability grants potential attackers control over speculative execution, which can lead to devastating information disclosure and nefarious data breaches. It is crucial for software developers, hardware vendors, and systems administrators to be aware of this vulnerability, implement necessary mitigation measures, and keep an eye out for updates from the CPU manufacturer.

Timeline

Published on: 08/08/2023 18:15:00 UTC
Last modified on: 08/27/2023 03:15:00 UTC